1. Field of the Invention
The present invention relates to an oscillation circuit. In particular, the present invention relates to an oscillation circuit including an astable multivibrator.
2. Description of the Related Art
An oscillation circuit is often configured with an astable multivibrator. A typical astable multivibrator charges and discharges two capacitors alternately to generate oscillation waveforms. A circuit for charging and discharging these capacitors is mounted in such the astable multivibrator. The voltages applied to the capacitors are fed back to the charge-discharge circuit, and the charge-discharge circuit charges and discharges the two capacitors in response to the applied voltages.
Oscillation frequency of the astable multivibrator is determined by the time required for charging and discharging the two capacitors. This charge-discharge time can be controlled easily by adjusting a charging current for charging the capacitor. Therefore, the oscillation frequency is often controlled by the charging current. The charging current can be used for controlling the oscillation frequency, and hence is referred to as a control current. In a VCO (Voltage Controlled Oscillator), a control current corresponding to a control voltage is generated and its oscillation frequency is controlled by the control current.
It is important that there is consistency between oscillation frequencies of a plurality of oscillation circuits. When a considerable number of semiconductor devices having the oscillation circuit are manufactured, large variation in the oscillation frequencies causes malfunctions in many semiconductor devices.
Japanese Unexamined Patent Publication JP-Sho-62-274913 (Patent document No. 1) indicates that variation in threshold voltages of MOS transistors in a circuit for supplying a control current charging the capacitors causes the variation in the oscillation frequencies. The patent document No. 1 discloses a voltage controlled oscillator which suppresses such a variation in the oscillation frequencies by canceling the variation in the threshold voltages of the MOS transistors.
Variation in the threshold voltages of MOS transistors that configures a feedback path for providing a feedback of the applied voltages to the capacitors to the charge-discharge circuit (for example, the MOS transistors that configures the inverter N3, N4 in FIG. 1 in the patent document No. 1) can also cause the variation in the oscillation frequencies. It is preferable in the oscillation circuit that the influences of the variation in the threshold voltages of the MOS transistors constituting the feedback path is eliminated and the variation in the oscillation frequencies is reduced.